It is known in the prior art that combustion of any type of fuel produces particulate or powders, this means solid particles containing both incombusted/partially combusted organic substances (BC-black carbon, soot; or OC-organic carbon), and inorganic substances such as salts and metal oxides, among which heavy metals, as Ni, V, Se, Cr, etc.).
As regards the impact of powders on human health, it is worthwhile stressing that clear epidemiological data until recently were not available. However general worries deriving from human physiology knowledge in particular were concerned and a more marked effect of the emitted particulate was expected, from particles having a lower diameter for example below 1 down to 10 manometer. It was in fact kept, qualitatively, that the specific danger given by the of submicron particle was higher owing to their inherent capability to come into strict and permanent contact with the human tissues (e.g.: lung alveoli) contiguous and permeable to the blood circulation system (FIGS. 5.18, page 354, Aerosols Chapter 5). In other words, it was believed that the smaller the solid particle, the higher the danger, in addition of the inherent dangerousness of the type of the emitted substance released in the flue gases of the combustor.
As regards to the environmental aspect, fume powders have been taken into consideration for their potential effect both on the earth warming and on air and earth pollution when related to dangerous suspected substances.
Concerns on the potential effects on human health gave rise to several toxicological and environmental studies, in exponential growth in the last decade. However the epidemiological studies in progress have not influenced the regulations in force on fume emissions for that it relates to the limit of the total allowed number of submicron particles. Likewise any distinction as to their intrinsic dangerousness is still absent, the present regulations being still and only based on the consolidated knowledge of the harmful effects of the emitted substances. For example, the rule still maintains emission limits expressed as total weight (indistinct) of the emitted PM 10 particulate (acronym indicating the particulate having <10 μm size), and in detail, for the most strict regulations, values <10 mg/Nm3 of emitted fumes, dry basis. Both the characterization methodologies and the values obtained therefrom, do not seem to take into account the evaluation of the danger.
The use of the PM 2.5 indicator (total particles having a diameter <2.5 micrometer), representative of the greatest importance to be attributed to the portion of the smaller particles inside the total particulate, has found only a theoretical application in several toxicological studies and no relevance in national regulations.
But recently, more and more numerous and reliable toxicological data, made available by the literature of the field, have allowed to draw clear conclusions on the dangerousness of the particulate and on the greatest impact of the smallest particles (greater epidemiological significance of the indicator PM 2.5), and above all on the significant correlation between the impact on human health and emissions both of BC and OC, and heavy metals as nickel, vanadium, selenium, lead in the order.
At the recent PCC conference of Pittsburgh (Pittsburgh Coal Conference) of 2012 a final study of DOE (US Energy Department) outlines clear cause-effect correlations, with a shocking indicator: 2.5 years of reduction in life expectation. The study has an emblematic title: Why did it take so long to find out?. This study reflects voluminous report presented by EPA (Environmental Protection Agency) at the Congress of the United States, in March 2012 (ref.: EPA-450/S-12-001; on line: http://www.epa.gov/blackcarbon).
The problem potentially concerns all the uses of fuels, from autotraction to power stations, to home heating with biomasses, etc.
Certainly this will give rise to more and more specific regulations in the time, much diluted in the time, because of non-availability of technological solutions at present. As a matter of fact the submicron particulate is outside the “technology limit” of the available industrial technologies, i.e. it escapes the known industrial techniques of flue gases post-treatment that are effective above 1 μm particle size. Even a series of more unit operations on flue gases, such as for example filtration with sleeve filters followed by electrostatic filtration and by wet electrostatic filtration, are effective (efficiency >80%) only above 1 μm particle size. Very strict cleanings of combustion fumes can take place therefore with low unit efficiencies and consequently with very significant costs and complexity.
Likewise and in parallel, the activities of development of power production technologies without release of greenhouse gas (that is, including the CO2 capture) conflict with the technological problem of the influence of the particulate, residual in flue gases after post-treatments, on the efficiency of the CO2 capture systems. In the case of post-firing CO2 capture (capture from flue gases after combustion in air), the particulate heavily interferes, for example with molecules (amines) that are active agents in the CO2 capture. This reduces the process efficiency and thus increases both the investments requested and the parasitic consumption of the produced total power. In case of the developing technologies that modify the combustion process, for example by switching from air to oxygen to obtain concentrated CO2 in the fumes, the presence of the particulate requires special and expensive fume cleaning operations, so to make then available a CO2 gas that can be fed to high efficiency compression rotary machines, without accumulation in the downstream systems conveying and reintroducing CO2 under the ground. The comparison between CAPEX (capital expenditure) and OPEX (operation expenditure) of a simple CO2 compression, with the relevant cost estimates of the industrial development plans that are under way (ex. Futuregen 1 and 2 in USA), and that must therefore include fumes cleaning, is quite evident.
Therefore recent evidences point to the effects both on the human health and on the environment of the submicron particulate, produced by the combustion plants with conventional technologies, and on the cost factors of the developing technologies with CO2 capture for reducing the greenhouse effect (target: integral impact reduced on the environment).
In consideration of the limits of the conventional technologies of fume cleaning, it was therefore felt the widespread need to intervene at the beginning on the combustion process itself, in order to avoid the formation and emission with combustion fumes of ultrafine (submicron) particulate.
As said above, the particulate is formed both of incombusted or partially combusted particles (organic particulate), and of inorganic particles (in particular metals).
In the prior art on combustion the concentration of the emitted particulate is generally expressed by weight (integral of the particle size weight distribution up to 10 μm, or up to 2.5 μm) referred to the volume unit of produced or emitted fumes. The weight integral value of the particle size distribution by unit volume of the fumes, below 1 μm particle size is not taken into consideration. And, even less, there is not even an indirect mention to the submicron numeral distribution of the ash particles in fumes. Some patents concerning combustion can for example be mentioned.
EP 1,616,129 in the name of the Applicant relates to a flameless combustion process to treat materials of various kind, for example waste, wherein the formation of cold zones in the reactor is reduced, the process being carried out under isothermal or quasi isothermal conditions, operating at temperatures higher than 1,300K and under pressures higher than the atmospheric one, the oxidant being fed in admixture with water, steam or recycling gas. In the patent it is stated that the combustion fumes contain very low TOC (Total Organic Carbon) amounts, of the order of ppm, as well as amounts of the same order of magnitude of fly ashes.
This patent does not give any indication on the numeral distribution of the particles in fumes.
U.S. Pat. No. 8,453,583 describes a combustion process using liquid, solid and gaseous fuels at temperatures in the range 1,300K and 2,500K, the comburent being oxygen, the combustion being carried out under “opaque” and infrared radiation, preferably under pressure and diluting oxygen with recycling fumes, and by adding to the inletting fuel of water/steam.
The combustion is exhaustive and produces fumes that do not contain organic particulate: TOC <1 ppm and soot not detectable. At the analysis carbon is below the sensitivity limits (<0.01% by weight). Also residual ashes in fumes, collected on a filter and screened with a release test in water, give negligible concentration values of incombusted products, near the analytical sensitivity limit (<0.01% by weight).
This patent does not give any indication on the submicron weight distribution of the ash particles in fumes and neither on the submicron numeral distribution, and even less on the distribution at particle diameters lower than 0.1 micrometer.
According to the teaching of these two patents, the incoming ashes are melted, coalesce in the melted state and are separated from fumes by collapsing on the refractored walls of the combustor. However, although the quantitative efficiency of the separation is high (>99%), the characterization of the fumes outletting the combustor carried out with a continuous process ELPI® type analyzer, that is an impactor collecting data on the weight and numeral distribution of the population of particles (inorganic particles in case of this teaching) with respect to their diameters, points out that the residual particle distribution is centered at values just above the micrometer (1-3 μm), the upper limit being above 7-8 μm, the distribution being open towards values lower than one micrometer.
As said, in terms of amounts by weight, the efficiency of the separation in the combustor is high, and the residual particle amount in fumes, determined by the techniques available for the evaluation of the PM 10 limit at chimney, is quite below this limit, even up to 1/10 with respect to the limit (EPA Method 201A—Determination of PM10 and PM 2.5 Emissions from Stationary Sources).
It has been found by the Applicant that the fraction of submicron population in fumes is almost unaffected in the processes described in these two patents.
EP 2,220,198 describes a combustion process using a flameless and isothermal combustor to which compounds having a melting temperature lower than or equal to 1450K are added. In this way in fumes the concentrations by weight of the particles having a diameter lower than 2.5 μm (PM 2.5) are reduced to values of about 10 μg/Nm3.
Also this patent does not give any indication on the numeral distribution of the particles, lower than 1 μm in fumes.
EP 2,227,521 relates to a combustion process in a flameless and isothermal combustor wherein compounds having a melting temperature lower than or equal to 1450K and sulphur, or sulphur-containing compounds, are used as additives. In the combustion fumes, at the same time of PM 2.5, also basic ashes are reduced, the latter being transformed into compounds that are no longer aggressive towards the walls of the combustor and for the downstream thermal recovery equipments.
Also this patent does not give any indication on the numeral distribution of the particles in fumes outletting the combustor.
However, the experimentally detected typology of the particle size numeral distribution (more significant than that by weight) obtained by the ELPI® analyzer, as said above, does not substantially change its typical characteristic profile.
EP 2,227,523 relates to a combustion process carried out in an isothermal and flameless combustor, wherein as additives sulphur or sulphur-containing compounds are used. In this way the outletting combustion fumes are no longer aggressive for those parts of the equipments that come into contact with the fumes.
Also in this patent there is no indication as to the submicron particle size numeral distribution of ashes in fumes.
The need was therefore felt to have available a process that, while maintaining the performances of the above described combustion processes, would allow a significant reduction of the portion of the particle size numeral distribution of fuel inorganic ashes below 1 μm for at least of about one half, up to about 6-8 times less that initial, and reaching a particle size numeral distribution close to that of the natural background, with clear advantages for human health, for 002 capture processes and with an increased efficiency of the plants used for fume cleaning, combined with a substantial elimination of the organic particulate (incombusted products) in fumes.